2 KiB
The Go for loop is similar to — but not the same as — C's. It unifies for and while and there is no do-while. There are three forms, only one of which has semicolons.
- Like a C
for:for init; condition; post { } - Like a C
while:for condition { } - Like a C
for(;;):for { }
Short declarations make it easy to declare the index variable right in the loop.
sum := 0
for i := 0; i < 10; i++ {
sum += i
}
If you're looping over an array, slice, string, or map, or reading from a channel, a range clause can manage the loop.
for key, value := range oldMap {
newMap[key] = value
}
If you only need the first item in the range (the key or index), drop the second:
for key := range m {
if key.expired() {
delete(m, key)
}
}
If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first:
sum := 0
for _, value := range array {
sum += value
}
The blank identifier has many uses, as described in a later section.
For strings, the range does more work for you, breaking out individual Unicode code points by parsing the UTF-8. Erroneous encodings consume one byte and produce the replacement rune U+FFFD. (The name (with associated builtin type) rune is Go terminology for a single Unicode code point. See the language specification for details.)
The loop
for pos, char := range "日本\x80語" { // \x80 is an illegal UTF-8 encoding
fmt.Printf("character %#U starts at byte position %d\n", char, pos)
}
prints
character U+65E5 '日' starts at byte position 0
character U+672C '本' starts at byte position 3
character U+FFFD '�' starts at byte position 6
character U+8A9E '語' starts at byte position 7
Finally, Go has no comma operator and ++ and -- are statements not expressions. Thus if you want to run multiple variables in a for you should use parallel assignment (although that precludes ++ and --).
// Reverse a
for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 {
a[i], a[j] = a[j], a[i]
}